Color-control grid structure for cathode-ray tubes



C. S. NUNAN Feb. 24, 1959 2,875,363 COLOR-CONTROL GRID STRUCTURE FOR CATHODE-IKAY TUBES 2 Sheets-Sheet 1 Filed Dec.

INVENTOR. 534/6 6: A u/ww United States PatntjfOfli in 2,875,363 it Patented Feb. 24, 1959 COLOR-CONTROL GRID STRUCTURE FOR CATHODE-RAY TUBES Craig S. Nunan, Palo Alto, Calif., assignor to Chromatic Television Laboratories, Inc., New York, N. Y., a corporation of California a Application December 12, 1955, Serial No. 552,532

4 Claims. c1. 313-92) This invention relates to cathode-ray tubes of the type adapted for. reconstructing images in colors closely 'approximating those of an original and, in particular, it is directed to the so-called color-control grid structure for use in such tubes. a

a In one form of cathode-ray tube for producing images in color, the target upon which the imageis caused to appear is formed of a multiplicity of phosphor-coated strips positioned adjacent to each other. Such coatings are of selected phosphor composition that when impacted by a scanning cathode-ray beam light which is observable.

Tubes of a type wherein the images are developed by a scanning cathode-ray beam sharply focused upon the target in a region. immediately adjacent to the target under the control of a color-control grid structure are known as the post-deflection focusing (PDF) variety. A color-control grid of this sort comprises a multiplicity of tautly stretched linear conductors or wires arranged to extend generally parallelto the phosphor-coated strips of the tube target. Where the target comprises a series of phosphor-coated strips adapted to produce light in the color sequence red, green, blue, green, red, green, etc., itis usually desirable to electro-optically center the linear conductors of the color-control grid over the phosphor strips to produce the red and blue portions of the image, leaving the phosphor strips to produce green light electro-optically centered relative to the aperture formed between adjacent conductors of the color-control grid structure. i a

The color-control grid structure is preferably formed of a series of tautly stretched linear'conductors or wires, each of which is secured or attached at its opposite ends to an electrical conductor secured to or forming a part of a frame member having a central window area or opening of a size which closely approximates that of the image area to be developed. This support frame is usually held from or by the tube wall in a position ad jacent to the phosphoncoated target. The color tubes, to which this invention is particularly related,provide a color-control grid structure of a type wherein adjacent linear conductors of the grid are maintained electrically insulated fromeach other and alternate conductors of the color-control grid are electrically connected. 'An electron source to develop a scanning cathoderay beam is provided in the tube, with the developed beam directed toward the target area through the color-control grid.

' The target areaand phosphor-coating is provided with an electron-permeable electrically conducting coating through which the scanning cathode-ray beam penetrates to impact the phosphor coating. A suitable voltage is applied to the color-control grid (usually anode voltage) andalso to the electron-permeable coating-"with the Jul latter made substantially more positive relative to the electron source to provide acceleration of the cathode-ray beam in the region between the color-control grid and the final target and also to develop (using the grid and the coating as the electrodes of a multiplicity of cylindrical electronic lenses) suitable focusing fields to bring the beam to a sharp focus at the target. Where alternate conductors of the color-control grid are electrically connected to each other and adjacent conductors of the colorcontrol grid are electrically insulated from each other, it is possible to change the relative potential on adjacent conductors so that the beam, as it passes through the grid to impact the target, is switched or deflected between adjacent phosphor-coated strips.

The color switching provides for changing the color of light instantaneously resulting from cathode-ray beam impact. However, with a single beam color tube, to produce an operation which will adapt itself to the production of the color television image. under presently existing transmission standards as adopted by the Federal Communications Commission, where the color informationis transmitted as a modulation, by two color signals called the I and Q signals, of a color sub-carrier frequency set at 3.58 me. (actually 3.579545 me.) it is desirable to apply this color sub-carrier as radio frequency (hereinafterreferred to as R. F.) modulation upon the color-control grid of the tube whereby the color in} stantaneously reproduced is switched between the selected phosphor strips at a rate coinciding with that of the color sub-carrier frequency. Various ways to apply such R. F. modulation to the color-controlgrid have already been suggested in the prior art, but because of heating effects resulting from the R. F. powerless the conductors of the color-control grid tend to expand. Frequently the expansion is enough (if there is a temperature rise-of as much as C.) to change the length of the conductors sufficiently to introduce substantialdetensioning of theoriginally tautly stretched linear conductors of the grid. It will be appreciated, of course, that the heat developed resulting in temperature rise in the conductor is proportional to the square of the R. F. currentflowing in them, necessi; tated by the need to deflect the impacting cathode-ray beam over thedifferent phosphor strips during the scan ning operation to produce all colors of the selected tricolor. The primary wayby which the heat developed within the conductors of the color-control grid can be dissipated is by radiation. Consequently,the tendency for the conductors of the color-control grid to expand and to become detensioned is a function of the thermal emissivity'of the conductor surface and of the conductor di ametero This is because of the fact that, generally speaking, the conductors of the color-control grid are chosen of a material selected primarily for its strength (to permit taut stretching) and because for reasons of tube fabrication it is desirable that. the metal selected for the conductors shall have a coefiicient of thermal expansion which is higher than 'th'at of the'supporting frame and the material from which the phosphor target plate is formed; l i 1 It is already known in the art that a type of colorcontrol grid of the described character permits the grid conductorsto loosen during the tube finishing heat cycles due to greater expansion than the support frame, but again to tighten and to become properly stressed, often to stressed conditions of as much as 50,000 lbs. per square inch and more, upon a return of the tube to a condition of room temperature. However, it is known that where there is a temperature differential of only slightly more than 100 C. between the linear conductors of the colorcontrol grid and the supporting frame this stress is reduced tof substantially a zero value by conductor ex'panl v g H 2,875,368

sion. Where the conductors of the color-control grid are heated due to the R. F. current flowing in them to deflect the scanning cathode-ray beam over the phosphor strips, any tendency for the conductors of the colorcontrol' grid to become detensioned due to expansion is such that the field exerted upon the conductors by the high voltage maintained upon the electron-permeable coating on the phosphors with respect to the color-control grid tends to pull the conductors of the grid out of plane, which disturbs at once the color purity of the color image. 7

If there is a collapse of the post-deflection focusing field, due to any sparking resulting from sagging conductors contacting each other, for instance, the conductors tend to snap back and occasionally short to an adjacent conductor and cause permanent damage. Even with half detensioning the vibration resonant frequency of the linear conductors of the color-control grid is reduced by a factor represented by the /2 and the vibration amplitude for any given energy impulse is increased by the same factor of the /2.

In the manufacture of any tube of the sort herein being considered for recreating color television images it is desirable in order that absorption and scattering of the primary electrons may be minimized and, similarly, the emission of secondary electrons be minimized and that the moire effect be reduced, that the conductor size of the color-control grid be held at a minimum value.

The present invention seeks to provide a solution of the problem wherein the linear conductors of the colorcontrol grid which are adapted for connection at either end, are prevented, to the maximum extent possible, from sagging and expanding to a point where the quality of the resultant image is seriously affected. To achieve this objective, it becomes one of the main purposes of this invention to provide a color-control grid structure for a cathode-ray tube to produce color television images in which the thermal emissivity of the color-control grid conductors is substantially increased,

It is a further object of the invention to provide for heat reduction by making the linear conductors of the color-control grid contact or connect to the thermal looking in the direction of the arrows and is for the purpose of showing the general arrangement of the grid conductors of the color-control grid relative to the supporting frame;

Fig. 3 is a greatly enlarged sectional view taken through one of the linear conductors of the color-control grid;

Fig. 4 is also a greatly enlarged sectional view of a modified form of linear conductor for a color-control grid, each utilizing the principles of the invention herein to be'described; and,

Figs. 5 and 6 are two series of curves to illustrate schematically the relationship of various forms of conductors plottingin Fig. 5 the average temperature rise against conductor diameter, particularly for a conductor connected at one end only, and in Fig. 6 a'somewhat similar curve is plotted to represent temperature rise in the conductors against selected positions along the conductor length for various averaged temperature rises, plotting conditions for conductors which are connected at both ends and at one end only. j

Referring now to the drawings for a further understanding of the invention, a schematic representation of a cathode-ray tube envelope is shown at 11. There is contained within the neck 13 of the tube envelope a suitable electron gun construction 15, schematically designated by the various electrodes shown at the left end of the tube neck, looking at the drawings. A cathoderay beam, conventionally represented by the dot-dash line within the tube, is developed by the electron gun 15 and is directed toward a target element schematically represented at 17 immediately adjacent to the window area 1 9,through which the produced image is viewed The" target area 21 is coated on its surface'with phosphor coating arranged in strip formation, as. is well known in the art. On the side of the phosphor coating toward the electron gun there is provided an extremely thin conducting film of some material such as aluminum or other appropriate metal, which is electron permeable points at which the R. F. voltages are applied at each end, and still further, it is an object of the invention to tively, may be 50,000 lbs. per square inch and higher,

while at the same time providing a high degree of heat radiation. In a still further refined form of the invention electrical conductivity is sought for at least the outer surface. of the conductor which is greater than that of the internal core having the higher tensile strength.

Other objects of the invention are to provide 3,.CO1OI1 control grid wherein the tendency of the conductors of the grid to become deformed or to sag due to detensioning resulting from heating under conditions ofapplication of high frequency switching voltages is reduced to a minimum value.

Other objects of the invention are to improve the colorcontrol grid for a-color image producing tube to an extent such that fidelity of image production, color puria ty and freedom from color contamination is greatly improved. 7

Other objects and advantages of the invention will becomeapparent from a consideration of the following description of a practical embodiment thereof, considered in conjunction with the accompanying drawings and the appended claims.

The accompanying drawings illustrate by. Fig. l a schematic representation of a color television tube having. a color-control grid positionedand supported therein relative: to the phosphor-coated target.

. Fig. 2. represents, in schematic arrangement, .a section:

taken through the tube of Fig. lalongsth'e line 2-2 and to which a voltage can be applied uniformly over the target surface. Adjacent to the target, as more particularly shown by the sectional view of Fig. 2, there is supported a color-control grid in the form of a multiplicity of'parallelly positioned and tautly stretched linear conductors 23. In the illustrated form of the invention, the linear conductors are shown as extending over the edge of the target surface 17, which is of insulating ma te'riaLsuch as glass. These conductors are arranged to be secured to connecting members 25 and 27 which are maintained in electrically insulate-d relationship with respect to each other by an appropriate spacer, such as the ceramic element 29. The conductors of the grid are maintained in appropriately spaced relationship from the phosphor coated target surface 21 by spacer elementsd], in well known fashion. Together, the connecting mem-.

b ers 2 5 and 27 form a frame which may be secured, as

schematically illustrated by the sectional view of Fig. 2, by brackets 33, to the wall of the tube or, if desired, the brackets may be formed into and become an integral part of the tube wall. The essential feature is that the conductors of the color-control grid are supported and secured at opposite ends to a suitable framework with alternate conductors secured to the same electrical section of the framework, and adjacent conductors held in electrically insulated relationship with respect to each other but likewise electrically secured together on an appropriateportion ofthe framework.

The precise arrangement of the framework isnot im portant tothe invention and various modifications of the supporting frame may be made. If desired, the supporting frame may be on the beam-impacting side of the target, as wellas the viewing side of the target. The

linear conductors when secured at opposite ends to a suitable support are'subjected to the application ofsuit'able R; F."potentials-thereon, supplied byway of connections schematically indicated at 35 and 37 and 35' and 37' protruding out through the tube wall. Suitable R. F. voltages at the selected color sub-carrier frequency when so applied efiect changing relative potentials of adjacent conductors with respect to each other. I

In the normal tube operation-the scanning cathoderay beam, as represented by the dot-dash line, is appropriately deflected in a bi-directional pattern by the electromagnetic deflecting coils schematically represented as 39 for providing the line or horizontal deflection, and the coils 41 for providing the vertical or field deflection, in accordance with well known practice. In the operation of the .tube the linear conductors 23 forming the color control grid structureare alined into substantial parallellism with the strips of the phosphor coated target, and, as already explained, with the color sequence of the light effects produced by the cathode-ray beam impacting the target being such that as the scanning beam moves from strip to strip it produces light in the color sequence red, green, blue, green, red, green, etc., one set of the alternate conductors may be electro-optically cen tered over the phosphor strips to produce red light, for instance, while the other set of alternate conductors will be electro-optically centered over the strips to produce blue light, as is the known practice. This leaves the phosphor strips which develop green light as centered. relative to the aperture formed between adjacent conductors of the color-control grid.

In the scanning operation with the R. F. voltage applied to the linear conductors 23 of the color-control grid it is usually preferable to cause the scanning operation in the line or horizontal deflection path of the cathode-ray beam to extend along the length of the phosphor-coated strips and the linear conductors of the color-control grid so that as theR. F. voltage is applied to the color-control grid conductors the scanning cathode-ray beam oscillates at the sub-carrier frequency between limiting values such that at the crest of the oscillation in one direction the resultant light color is red and at the crest of the oscillation in the other direction the resultant light color is blue, with the green color light being produced each time oscillation goes through its nodal point andfor appropriate distances either side thereof. This type of operation is also well known, but is recited herein for illustrative purposes As the scanning beam is then directed through the apertures formed by the linear conductors of the color-control grid to reach the target the colorcontrol grid normally has providedupon it D. C. potentials substantially corresponding to that of the second anode of the electron gun from which the cathode-ray beam is emitted, theR. F. voltage is superimposed on this D. C. voltage. The conducting film of the target then customarily has applied to it a potential which is of theorder of four times that which is applied to the linear conductors of the color-control grid with respect to the cathode element of the electron gun.

Fig; 3 is presented schematically to illustrate one form of linear conductor for the color-control grid. This structure is one having a central core41, preferably of a metal possessing high tensile strength. Illustratively, a conductor of this sort may be a wire of the chromium nickel variety or one of the chromium stainless steel variety, or it may be a steel or other metal Wire which has adequate tensile strength so that when the conductors of the grid may be stressed between the points of securement they may be. stressed with a pulling pressure of asmuch as 50,000 lbs. per square inch in their non-heated state. This stressing is important in the construction in order to reduce conductor vibration and sagging. i With the application of the radio frequency switching voltage to the linear conductors of the color-control grid,

and with this voltage being applied at each end, heat radiation is minimized. However, occasionally one of the conductors of the grid will fail to make contact at one end or the other, with the result that the high he quency voltage is fedat only one end, and for such coriditions the conductor temperature tends to rise more rapidly than would be the case for contact at both ends.

A blackened outer surface, shown at 4,3, is formed upon the outer surface of the high tensile strength central core by blackening, coating, oxidizing or in any other desired fashion. This coating. may be formed of various substances. It is, however, particularly adapted readily to radiate" heat and the closer it approaches black body conditions the better the operation.

In the modification of Fig. 4 the central conductor 41 is also shown as a conductor of high tensile strength, such as stainless steel, chromium steel, chromium nickel or the like. There is provided about this outer conductor, as will herein later be explained, a covering or plating of silver, copper or some other material of usually lesser tensile strength than the internal core, but which oifers less electrical resistance and higher conductivity to the flow of electrical current than the central core, and it is particularly a material which can readily be blackened; The coating of high conductivity is indicated in Fig. 4 by the schematically represented layer 45, while the blackened surface area provided by the outer coating is shown at 43'.

Making reference now to the curves of Fig. 5, for an understanding of a practical operating device, the ordinate is plotted as average temperature rise in degrees centigrade as against the wire diameter in mils plotted as the abscissa. Various curves are shown for difi'erent emissivities. Illus tratively, curve 59 is representative of a No. 347 stainless steel wire having an emissivity of 0.05, this value being typical of a wire in an unblackened state or condition. Curve 53 is a curve for a similar type of wire having an emissivity of 0.10. Curve 55 is also a similar type of wire but its emissivity is 0.20. I The same general effect can be achieved from a Wire having acentral core of stainless steel which is silver-plated to a thickness of approximately 2% of the wire diameter. Where the silver plating has an emissivity of 0.05, for instance, the curve almost duplicates the stainless steel wire having an emissivity of 0.20. Curve 57 shows the conditions for a stainless steel wire having an emissivity of 0.40, or a stainless steel wire, silver-plated as above explained, with a szurface thermal emissivity of 0.10. Curve 59 is indicative of a stainless steel wire having an emissivity of 0.80, or a stainless steel Wire, silver-plated, as above explained, and having a thermal emissivity of 0.20. Curve 61 is indicative of a silver-plated wire having an emissivity of 0.40 and, lastly,

curve 63 is a similar type of wire but having an emissivity of 0.80.

The dotted line indicated at a temperature of 104" represents the temperature rise which reduces the wire stress from a value of the order of 50,000 lbs. per square inch to zero. In each of the instances above explained, the Wire has been blackened to produce a higher degree of thermal emissivity. Values for other operating con ditions may be determined by interpolation.

To form a linear conductor of the type considered by the curves of Fig. 5, illustratively, the chromium nickel or chromium stainless steel Wire, after having been cleaned and dried, may be immersed in a molten solution of sodium dichromate at a temperature generally within the range of 730 F. to 750 F. for a period of about 15 minutes. The blackening process resulting produces a smooth oxide film on the wire outer surface which is approximately 10 micro-inches thick. This film has great stability and substantially the same corrosion resistance as the base metal. The blackening, of course, increases the thermal emissivity coetficient.

The linear conductors or wires also may be blackened in other Ways. Illustratively, the silver Wire or the silver coating on the stainless steel wires may be blackened by passing the wire through ammonium sulfide which forms silver sulfide, which is black, on the outer surface. Vars ious other methods .for blackening the wires or'linear conductors and thereby increasing the emissivity are to paint, as it were, the finished grid with a mixture of potassium'silicate and lampblack, for'instance. Still other methods of blackening may be; resorted to, the specific form of blackened body not constituting this invention, but rather, cooperating with the other components to pro vide an increase in the radiation. It is well known that blackened wire, per se, is available on the open market. Likewise, silverplated wires are available on the open market and the sulfide or other treatment to blacken the silver is likewise a known process.

Therefore, the invention as here represented, comprises, in effect, the recognition that the blackening increases the thermal emissivity to the point where wire detensio-ning is avoided and greatly improved color fidelity achieved because the wire detensioning is controllable which, so far as is known, has not heretofore been achieved.

As can be recognized from the curves of Fig. 5, if the conductors are assumed, as there depicted, to be connected at one end only it is evident that a conductor which is 5 mils in diameter with a thermal emissivity of 0.06 will be completely detensioned by the exciting R. F. A similar condition would occur in a 5.25 mil conductor having an emissivity of 0.05. However, still referring to the curves of Fig. 5, it will be observed that if the thermal emissivity of the linear conductor is increased, for a stainless steel conductor, to a value of 0.20 by blackening the occasional conductor of the color-control grid which is connected at only one end will be only half detensioned by reason of the R. F. heating, with a consequent reduction of 3.0% in resonant frequency.

The curves of Fig. 5 also indicate that for a stainless steel conductor having an emissivity of approximately 1.00, as increased by blackening and as would be represented by the point 65 on the curves of Fig. 5, it is possible to use a 3 mil diameter conductor for the grid and the occasional conductorwhich is connected at only one end will then only be half detensioned by reason of R. F. heating.

It will, of course, be appreciated that in most instances as a practical matter emissivity of unity is extremely difficult to approach. To this end the curves of Fig.5 indicate that in a conductor having a central core of material of high tensile strength, which usually has a relatively high electrical resistance as compared to silver or copper and, therefore, would heat more rapidly (heating being a function of the square of the current and the resistance) heating is reduced if the central core is v covering of approximately 2% of the wire diameter or to a thickness of 0.06 mils and then blackened, a thermal emissivity of 0.30, which is an easily obtainable value,

and which would be represented, for instance, also by the point 65 on the curves of Fig. 5, will show that an occasional wire which is connected at only one end will only be half detensioned by R. F. heating effects.

In connection with the curves represented by Fig. 5 the values have been chosen for a so-called 22 inch Chromatron tube operating with the conducting film on the phosphor coated target maintained at a potential V of 22,000 volts positive relative to the tube cathode and with the pitch between the adjacent conductors of the color-control grid approximately 25 mils, and with a spacing of approximately 70() mils between the colorcontrol grid and the phosphor-coated target which pro vides a target image area of approximately x In the curves of Fig. 6 similar conditions have been wire for various averaged temperature rises. In this series of curves the curve 67 represents an average temperature rise of 47.9. (3.; curve 68 assumes a temperature rise of 28.3 0.; curve 69 assumes anaverage temperature rise of 17.8 C. and curve 79 an average temperature rise of 8.2 C. These curves represent the conditions where the linear conductors of the grid are fed with the R. F. from each end. The curve also shows by the continuations represented by the extensions 71, 72, 73 and 74 linear conductors fed at only one end where the curve 71 indicates an average temperature rise of l20.4 C. Curve 72 provides an average temperature rise of 793 C. Curve 73 shows an average temperature rise of 54.4 C. and curve 74 shows the average temperature rise at 279 C.

Various other forms, of course, may be determined by suitable interpolations. The significant factors are, however, that conductors fed at each end show substantially less temperature rise than conductors fed at one end only and, therefore, it is desirable in practicing the invention to reduce the averaged temperature rise by connecting the conductors to the R. F. supply at each end which produces an improvement of approximately the factor of 3.

Having now described the invention, what is claimed is:

l. A color-control grid structure for use in a cathoderay tube to produce images in color on the phosphorcoated target area thereof which comprises a support frame having a central window opening of an area approximating that of an image to be produced, a plurality of tautly stretched parallelly positioned linear conductors spanning the frame opening, each of the said conductors being adapted to be secured at each end to terminal means to apply both operating and radio frequency switching voltages thereto, and each linear conductor comprising a central metal core of high tensile strength having a blackened outer surface to provide high thermal emissivity and thereby substantially reduce conductor detensioning due to longitudinal expansion resulting from radio frequency power loss.

2. A color-control grid structure for use in a cathode ray tube to produce images in color on the phosphorcoated target area thereof which comprises a support frame having a central window opening of an area approximating that of an image to be produced, a plurality of tautly stretched parall'elly positioned linear conductors spanning the frame opening, each of the said conductors being adapted to be secured at each end thereof to terminal means to apply both operating and radio frequency switching voltages thereto, and each linear con- 3. A color-control grid structure for use in a cathode ray tube to produce images in color on the phosphorcoated target area thereof which comprisesfa support frame having a central window opening of an area approximating that of an image to be produced, a plurality of tautly stretched parallelly positioned linear conductors spanning the frame opening, each of the said conductors being adapted to be secured at each end thereof to terminal means to apply both operating and radio frequency switching voltage thereto, and each linear conductor comprising a central core of metal of high tensile strength and an external covering metal of a' thickness only a minor fraction of the core diameter assesstiguous therewith throughout the core length and having an electrical resistance and tensile strength-substantially less than that of the centralcore and an outer coating of high thermal emissivity intimately bonded with the low resistance conductor throughout its length so that detensioning of the composite linear conductor due to radio frequency power loss resulting from radio frequency voltage application thereto is reduced.

4. A color-control grid structure for use in a cathoderay tube to produce images in color on the phosphorcoated target area thereof which comprises a support frame having a central window opening of an area approximating that of an image to be produced, a plurality of tautly stretched parallelly positioned linear conductors spanning the frame opening, each of the said conductors being adapted to be secured at each end thereof to terminal means from which both operating and radio frequency switching voltage is applied thereto, and each linear conductor comprising a metal core of high tensile strength and a metal covering contiguous therewith throughout the conductor length, the metal covering 15 2,725,617

having an electrical resistance substantially less than that of the high tensile strength central core and an outer coating of high thermal emissivity intimately bonded with the low electrical resistance conductor for readily radiating heat developed by radio frequency exciting currents and thereby reducing detensioning of the linear conductors due to radio frequency power loss.

References Cited in the file of this patent UNITED STATES PATENTS 2,417,459 Eitel et al Mar. 18, 1947 2,677,070 Milligan Apr. 27, 1954 2,691,116 Allwine Oct. 5, 1954 Sternberg Dec. 6, 1955 

